Fluoropolymer members of fluoroplastic resins and analogues are of great interest for their mechanical properties and resistance against heat, weather, and chemicals. Since fluoroplastic resins are very expensive, they are often used as thin film sheets which are joined to supports of less expensive material which are responsible for air tightness and strength. Typical of such composite products are weather-resistant sheets.
Fluoropolymer members of fluoroplastic resins and analogues, however, are difficult to form composite products because they are difficultly adhesive due to their characteristic chemical stability. Efforts have heretofore been made for rendering the surface of fluoropolymer members hydrophilic for improving their adhesiveness. Prior art approaches include mechanical roughening, chemical etching, sputtering, flame treatment, and plasma or corona discharge treatment on the surface of fluoroplastic resin sheets. None of these approaches have gained sufficient bond strength.
Among the conventional approaches, the plasma surface treatment is an attractive surface treatment method for fluoropolymer members since it is clean and offers a high degree of freedom for surface modification. Usually, fluoropolymer members such as fluororesin sheets are modified by plasma discharge treatment to provide a hydrophilic surface suitable for adhesion. This treatment is often carried out in a gas atmosphere containing oxygen, argon or nitrogen or a gas mixture of oxygen and carbon tetrafluoride. The conventional plasma discharge treatment under such atmospheres, however, could not render the fluoropolymer member surface fully hydrophilic, thus failing to achieve satisfactory adhesion.
Another known method for obtaining hydrophilic fluoropolymer member surfaces is a low-pressure glow discharge process using helium gas as disclosed in J. R. Hall, C. A. L. Westerdahl et al, J. Appl. Polym. Sci., 16, 1465 (1975). The method could render fluoropolymer member surfaces hydrophilic, but not to an extent enough for adhesion, and the method remained practically unacceptable.
In addition, since low-pressure glow discharge plasma is generally carried under low pressures of less than about 10 Torr, this method requires a vacuum apparatus of a high capacity for commercial application, entailing increases in investment and operating cost. If objects to be treated contain much moisture and volatile plasticizers, these components would vaporize and leave the object surface in a vacuum atmosphere, disturbing the plasma treatment from achieving the intended function. Moreover, the plasma treatment of this type tends to produce heat and is thus undesirable to apply to articles of low-melting materials.
A commercially acceptable mode of corona discharge treatment also improves adhesion, but to an unsatisfactory level.